Natural gas liquefaction system including an integrally-geared turbo-compressor

ABSTRACT

According to one aspect of the present disclosure, a natural gas liquefaction system ( 100 ) is provided. The system comprises an integrally-geared turbo-compressor ( 150 ) with a plurality of compressor stages; a prime mover ( 160 ) for driving the compressor; a pre-cooling loop ( 110 ), through which a first refrigerant is adapted to circulate, wherein one or more first compressor stages ( 151 ) of the plurality of compressor stages are adapted to pressurize the first refrigerant; a cooling loop ( 130 ), through which a second refrigerant is adapted to circulate, wherein one or more second compressor stages ( 155 ) of the plurality of compressor stages are adapted to pressurize the second refrigerant; a first heat exchanger device ( 170 ) for transferring heat from a natural gas and/or from the second refrigerant to the first refrigerant; and a second heat exchanger device ( 180 ) for transferring heat from the natural gas to the second refrigerant. A further aspect relates to a compressor arrangement for a natural gas liquefaction system. A yet further aspect relates to a method of liquefying natural gas.

TECHNICAL FIELD

The present disclosure relates to systems and methods for liquefyingnatural gas. More specifically, the present disclosure relates to asystem for liquefying natural gas with an integrally-gearedturbo-compressor as well as to a compressor arrangement including anintegrally-geared turbo-compressor. Further, the present disclosurerelates to a method of liquefying natural gas with an integrally-gearedturbo-compressor.

BACKGROUND

Natural gas is becoming an increasingly important source of energy. Inorder to allow a cost-efficient transportation of the natural gas fromthe source of supply to the place of use, it is beneficial to reduce thevolume of the gas. Cryogenic liquefaction has become a routinelypracticed process for converting the natural gas into a liquid, which ismore convenient, less expensive and safer to store and transport.Transportation by pipeline or ship vessels of liquefied natural gas(LNG) becomes possible at ambient pressure, by keeping the chilled andliquefied gas at a temperature lower than liquefaction temperature atambient pressure.

In order to store and transport natural gas in the liquid state, thenatural gas is preferably cooled down to around −150 to −170° C., wherethe gas possesses a nearly atmospheric vapor pressure.

Several processes and systems are known for the liquefaction of naturalgas, which provide for sequentially passing the natural gas at anelevated pressure through a plurality of cooling stages where the gas iscooled to successively lower temperatures by sequential refrigerationcycles until the liquefaction temperature is achieved.

Prior to passing the natural gas through the cooling stages, the naturalgas is typically pretreated to remove impurities that can interfere theprocessing, damage the machinery or are undesired in the final product.Impurities include acid gases, sulfur compounds, carbon dioxide,mercaptans, water and mercury. The pre-treated gas from which impuritieshave been removed is then typically cooled by refrigerant streams toseparate heavier hydrocarbons. The remaining gas mainly consists ofmethane and usually contains less than 0.1% hydrocarbons of highermolecular weight, such as propane or heavier hydrocarbons. The cleanedand purified natural gas is cooled down to the final temperature in acryogenic section. The resulting LNG can be stored and transported atnearly atmospheric pressure.

Cryogenic liquefaction is usually performed by means of a multi-cycleprocess, i.e. a process using two or more refrigeration cycles.Depending upon the kind of process, each cycle can use a differentrefrigerant, or alternatively the same refrigerant can be used in two ormore cycles. In a typical cryogenic liquefaction system, e.g. in theso-called APCI process, the natural gas is first cooled by a firstrefrigerant which circulates in a pre-cooling loop and is subsequentlycooled by a second refrigerant which circulates in a cooling loop.

In the pre-cooling loop, the circulating first refrigerant may becompressed, condensed, and expanded, in order to subsequently removeheat from the natural gas. In the cooling loop, the circulating secondrefrigerant may be compressed and cooled, in order to subsequentlyremove heat from the natural gas. However, driving two cooling loops(pre-cooling loop and cooling loop) is energy-intensive, cost-intensiveand space-consuming.

Accordingly, it would be beneficial to design and provide methods andsystems for liquefying natural gas that provide a better energyefficiency and consume less space.

SUMMARY

In light of the above, a natural gas liquefaction system, a compressorarrangement as well as a method of liquefying natural gas are provided.

According to one aspect of the present disclosure, a natural gasliquefaction system is provided. The system includes: anintegrally-geared turbo-compressor with a plurality of compressorstages; a prime mover for driving the compressor; a pre-cooling loop,through which a first refrigerant is adapted to circulate, wherein oneor more first compressor stages of the plurality of compressor stagesare adapted to pressurize the first refrigerant; a cooling loop, throughwhich a second refrigerant is adapted to circulate, wherein one or moresecond compressor stages of the plurality of compressor stages areadapted to pressurize the second refrigerant; a first heat exchangerdevice for transferring heat from natural gas and/or from the secondrefrigerant to the first refrigerant; and a second heat exchanger devicefor transferring heat from the natural gas to the second refrigerant.

An integrally-geared turbo-compressor according to embodiments describedherein includes at least one force transmission mechanism, particularlya gear, connected between two or more compressor stages of the pluralityof compressor stages.

According to another aspect, a compressor arrangement for compressing aplurality of refrigerants is provided. The compressor arrangementincludes: an integrally-geared turbo-compressor with a plurality ofcompressor stages; a first cooling loop, through which a firstrefrigerant is adapted to circulate, wherein one or more firstcompressor stages of the plurality of compressor stages are adapted topressurize the first refrigerant; and a second cooling loop, throughwhich a second refrigerant is adapted to circulate, wherein one or moresecond compressor stages of the plurality of compressor stages areadapted to pressurize the second refrigerant.

According to another aspect, a method of liquefying natural gas isprovided. The method includes: providing an integrally-geared turbocompressor having a plurality of compressor stages; driving thecompressor with a prime mover; circulating a first refrigerant throughone or more first compressor stages of the plurality of compressorstages; circulating a second refrigerant through one or more secondcompressor stages of the plurality of compressor stages; cooling atleast one of natural gas and the second refrigerant by heat exchangeagainst the first refrigerant; and cooling the natural gas by heatexchange against the second refrigerant

Further aspects, advantages, and features of the present disclosure areapparent from the dependent claims, the description, and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments. The accompanying drawings relate to embodiments of thedisclosure and are described in the following. Some embodiments aredepicted in the drawings and are detailed in the description whichfollows.

FIG. 1 is a schematic diagram of a typical APCI process for liquefyingnatural gas;

FIG. 2 is a schematic diagram of a natural gas liquefaction systemaccording to embodiments described herein;

FIG. 3 is a schematic diagram of a natural gas liquefaction systemaccording to further embodiments described herein;

FIG. 4 is an enlarged schematic view of a compressor arrangement for anatural gas liquefaction system according to embodiments describedherein;

FIG. 5 is a schematic diagram of a natural gas liquefaction systemaccording to further embodiments described herein; and

FIG. 6 is a flow diagram illustrating a method of liquefying a naturalgas according to embodiments described herein.

DETAILED DESCRIPTION

Reference will now be made in detail to the various embodiments of thedisclosure, one or more examples of which are illustrated in thefigures. Each example is provided by way of explanation and is not meantas a limitation. For example, features illustrated or described as partof one embodiment can be used on or in conjunction with any otherembodiment to yield yet a further embodiment. It is intended that thepresent disclosure includes such modifications and variations.

Within the following description of the drawings, the same referencenumbers refer to corresponding or to similar components. Generally, onlythe differences with respect to the individual embodiments aredescribed. Unless specified otherwise, the description of a part oraspect in one embodiment applies to a corresponding part or aspect inanother embodiment as well.

FIG. 1 shows a schematic diagram of a typical natural gas liquefactionsystem using the so-called APCI process. The shown process uses tworefrigeration cycles. A pre-cooling cycle 12 uses a first refrigerantand a cooling cycle 2 uses a second refrigerant.

The system, labeled 1 as a whole, includes the cooling cycle 2 includinga line formed by a gas turbine 3, which drives a compressor train. Thecompressor train includes a first compressor 5 and a second compressor 7in series for compressing the second refrigerant. An inter-stage cooler9 may be provided to cool the second refrigerant delivered by the firstcompressor 5 to reduce the temperature and the volume of the secondrefrigerant before entering the second compressor 7. The compressedsecond refrigerant delivered by the second compressor 7 may be condensedagainst air or water in a second condenser 11. The second refrigerant iscooled and partly liquefied by a heat exchange against a firstrefrigerant which circulates in the pre-cooling cycle 12.

The pre-cooling cycle 12 includes a line including a gas turbine 13,which drives a compressor 15. The compressed first refrigerant deliveredby the compressor 15 is condensed in a first condenser 17 against wateror air. The condensed first refrigerant is used to pre-cool the naturalgas down to −40° C. and to cool and partially liquefy the secondrefrigerant. The pre-cooling of the natural gas and the partialliquefaction of the second refrigerant are performed in a multi-pressureprocess, e.g. a four pressure process in the example shown in FIG. 1.

The stream of the condensed first refrigerant from the first condenser17 is delivered to a first set of four, serially arranged auxiliary heatexchangers to cool and partly liquefy the second refrigerant, and to asecond set of four, serially arranged, pre-cooling heat exchangers topro-cool the natural gas. A first portion of the compressed firstrefrigerant streaming from the first condenser 17 is delivered through apipe 19 to the first set of heat exchangers and is sequentially expandedin the serially arranged expanders 21, 23, 25 and 27 to four different,gradually decreasing pressure levels. Downstream from each expander, aportion of the expanded first refrigerant is diverted to a respectiveheat exchanger 29, 31, 33, and 35.

The compressed second refrigerant delivered from the second condenser 11may flow in a pipe 37 toward a main cryogenic heat exchanger 38. Thepipe 37 sequentially passes through the heat exchangers 29, 31, 33 and35, such that the second refrigerant is gradually cooled and partlyliquefied against the expanded first refrigerant.

A second fraction of the condensed first refrigerant from the firstcondenser 17 is delivered to a second pipe 39 and expanded sequentiallyin four serially arranged expanders 41, 43, 45, and 47. A portion of thefirst refrigerant expanded in each expander is diverted towards acorresponding pre-cooling heat exchanger 49, 51, 53 and 55,respectively. A main natural gas line 61 flows sequentially through saidpre-cooling heat exchangers 49, 51, 53 and 55, such that the natural gasis pre-cooled before entering the main cryogenic heat exchanger 38. Theheated first refrigerant exiting the pre-cooling heat exchangers 49, 51,53 and 55 is collected with the first refrigerant exiting the heatexchangers 29, 31, 33 and 35, and is fed again to the compressor 15,which recovers the four evaporated streams of first refrigerant andre-compresses the vapor.

The system shown in FIG. 1 includes at least one compressor driven by agas turbine 13 for compressing the first refrigerant, and at least onefurther compressor driven by a gas turbine 3 for compressing the secondrefrigerant. Accordingly, the energy-efficiency of the system shown inFIG. 1 is limited, and the two gas turbines 3, 13 consume a considerableamount of space.

A natural gas liquefaction system 100 in accordance with embodimentsdescribed herein is schematically shown in FIG. 2.

The natural gas liquefaction system 100 includes an integrally-gearedturbo-compressor 150 (also simply referred to as compressor 150) with aplurality of compressor stages which is configured to be driven by aprime mover 160, particularly by a single prime mover such as aninternal combustion engine or an electric motor. In other words, eachcompressor stage of the plurality of compressor stages of the compressor150 may be driven directly or indirectly by the prime mover 160. Atransmission mechanism 301, particularly a gear of the compressorincluding one or more gear wheels, and/or other transmission units suchas pinions, pulleys, toothed wheels etc. may be connected between theplurality of compressor stages of the compressor 150, in order to drivethe plurality of compressor stages into rotation. The driving force maybe provided by the prime mover 160, e.g. via a main driving shaftconnected to the integrally-geared turbo compressor.

By providing a compressor with an integral gear, the speed, the torque,and/or the direction of the rotational force provided by the prime movercan be changed as appropriate. For example, the rotational speed and/orthe torque of the impellers of the plurality of compressor stages can beindividually adjusted as appropriate. In some embodiments, thetransmission mechanism can include a gear train or a transmission. Theimpellers of the compressor stages may be mounted on respective shaftswhich may be driven into rotation by one of the transmission elements ofthe gear. For example, the gear may include at least one gear wheelwhich may drive one or more shafts into rotation. A pinion may bemounted on each of the shafts which may mesh with at least one gearwheel. Further, one or two impellers of the plurality of compressorstages may be mounted on each of the shafts.

In an integrally-geared compressor, at least one or more transmissionunits such as one or more gearwheels are connected between at least someof the plurality of compressor stages, so that the respective impellersof the compressor stages can be rotated at different rotational speeds.When a gear or another force transmission mechanism is connected betweenat least some compressor stages, the compressor stages can be providedon different shafts, which may be adapted to rotate with differentrotational speeds. For example, the impellers of the one or more firstcompressor stages may be rotated at different rotational speeds as theimpellers of the one or more second compressor stages.

For example, in some embodiments, one or more force transmissionelements such as one or more central gearwheels may be provided fordriving the one or more first compressor stages and the one or moresecond compressor stages at different rotational speeds. The one or morefirst compressor stages may be provided on different shafts as the oneor more second compressor stages.

In some embodiments, which may be combined with other embodimentsdescribed herein, a force transmission element such as one or morecentral gearwheels may be configured for driving two or more firstcompressor stages at different rotational speeds. In some embodiments,which may be combined with other embodiments described herein, a forcetransmission element such as one or more central gearwheels may beconfigured for driving two or more second compressor stages at differentrotational speeds. In some embodiments, the integrally-geared compressormay include a plurality of force transmission elements such as aplurality of central gearwheels, in order to drive each stage of theplurality of compressor stages at a desired rotational speed.

As is further depicted in FIG. 2, the natural gas liquefaction system100 includes a pre-cooling loop 110, through which a first refrigerantis adapted to circulate, wherein one or more first compressor stages 151of the plurality of compressor stages are adapted to pressurize thefirst refrigerant, and a cooling loop 130, through which a secondrefrigerant is adapted to circulate, wherein one or more secondcompressor stages 155 of the plurality of compressor stages are adaptedto pressurize the second refrigerant.

Each compressor stage of the plurality of first and second compressorstages may include a gas inlet, a gas outlet, and at least one impellerrotating on a respective shaft. The compressor stages may be axial orradial compressor stages.

The one or more first compressor stages 151 for pressurizing the firstrefrigerant may be directly or indirectly driven by the prime mover 160,e.g. via the transmission mechanism or gear of the compressor. The oneor more second compressor stages 155 for pressurizing the secondrefrigerant may also be directly or indirectly driven by the prime mover160, e.g. via the transmission mechanism or gear of the compressor 150.

According to embodiments described herein, a single integrally-gearedmulti-stage compressor driven by the prime mover 160 may be provided forpressurizing two or more refrigerants circulating in two or more coolingloops, e.g. in the pre-cooling loop 110 and in the cooling loop 130. Insome embodiments, the whole LNG liquefaction system may include a singleintegrally-geared compressor configured for pressurizing the two or morerefrigerants which are used for liquefying the natural gas.

The first compressor stages and the second compressor stages of thecompressor may be housed in a single compressor casing, e.g. in acompact and space-saving way. For example, a wall of a compressorhousing may enclose the first plurality of compressor stages, the secondplurality of compressor stages, as well as the transmission elements ofthe gear of the compressor which connect the driving shafts of thecompressor stages with each other.

By using an integrally-geared multi-stage compressor for pressurizingtwo, three or more refrigerants of the LNG liquefaction system, energyand space can be saved as compared to previously used systems whichincluded one or more separate compressors. An adjustment of therotational speeds of the compressor stages may still be possible,because the plurality of compressor stages are drivingly connected bythe integral gear of the compressor.

As is further shown in FIG. 2, the natural gas liquefaction system 100may further include a first heat exchanger device 170 configured fortransferring heat from natural gas to the first refrigerant and/or fromthe second refrigerant to the first refrigerant, and a second heatexchanger device 180 for transferring heat from the natural gas to thesecond refrigerant.

In some embodiments, the natural gas is adapted to be sequentiallycooled by the first refrigerant and by the second refrigerant. Thenatural gas may be guided through one or more first heat exchangers ofthe first heat exchanger device 170, where the natural gas may bepre-cooled by the first refrigerant, e.g. to a temperature below 0° C.,particularly −40° C. or less. The natural gas may subsequently be guidedthrough the second heat exchanger device 180, where the natural gas iscooled by the second refrigerant. The second heat exchanger device 180may be the main cryogenic heat exchanger of the system which isconfigured to cool the natural gas down to the liquefaction temperature.

In the schematic diagram of FIG. 2, the second heat exchanger device 180is depicted in a simplified way as a device which removes heat from thenatural gas flowing through a main natural gas line 61 and transfers theheat to the second refrigerant flowing through the cooling loop 130.

The first refrigerant circulating in the pre-cooling loop 110 may beused for pre-cooling the natural gas at a position of the main naturalgas line 61 upstream from the second heat exchanger device 180.Alternatively or additionally, the first refrigerant may be used forcooling the second refrigerant at a position of the cooling loop 130upstream from the second heat exchanger device 180.

In the embodiment depicted in FIG. 2, the first heat exchanger device170 includes a heat exchanger configured for pre-cooling the natural gasand a further heat exchanger configured for cooling the secondrefrigerant. The first refrigerant which leaves the first heat exchangerdevice 170 may be guided back to the compressor 150 to be re-compressedin the one or more first compressor stages 151 of the compressor.

In some embodiments, the second refrigerant which leaves the second heatexchanger device 180 may be guided back to the compressor 150 to bere-compressed in the one or more second compressor stages 155 of thecompressor.

In some embodiments, which may be combined with other embodimentsdescribed herein, the pre-cooling loop 110 includes a first condenser 17for removing heat from the first refrigerant after compression. Thepre-cooling loop may further include at least one expansion element (notshown in FIG. 2) for expanding the first refrigerant upstream from thefirst heat exchanger device 170.

The cooling loop 130 may include a second condenser 11 for removing heatfrom the second refrigerant after compression.

In some embodiments, which may be combined with other embodimentsdescribed herein, the first refrigerant includes a gas with a molecularweight of 35 or more, particularly 40 or more, more particularlypropane.

In some embodiments, which may be combined with other embodimentsdescribed herein, the second refrigerant is a mixed refrigerant, whichmay include a mixture including at least one or more of nitrogen,methane, ethane and propane.

In some embodiments, which may be combined with other embodimentsdescribed herein, at least one compressor stage of the plurality ofcompressor stages is provided with a movable inlet guide vane forautonomously regulating a flow entering in the at least one compressorstage. For example, each of the one or more first compressor stages 151may be provided with a respective movable inlet guide vane.

As is schematically depicted in FIG. 2, a single prime mover may beprovided for driving each of the one or more first and second compressorstages. In some embodiments, the prime mover 160 may be or include a gasturbine and/or a motor, e.g. an electric motor or an internal combustionengine. One or more gearbox elements of the integrally-geared turbocompressor may be connected between the prime mover, the one or morefirst compressor stages and/or the one or more second compressor stages.For example, at least some of the impellers of the one or more firstcompressor stages may rotate at a different rotational speed and beprovided on different rotary shafts than at least some of the impellersof the one or more second compressor stages.

A natural gas liquefaction system 200 in accordance with embodimentsdescribed herein is schematically shown in FIG. 3. The basic setup ofthe natural gas liquefaction system 200 is similar to the system shownin FIG. 2 so that reference can be made to the above explanations whichare not repeated here.

The natural gas liquefaction system 200 includes an integrally-gearedturbo-compressor 150 with a plurality of compressor stages which isconfigured to be driven by a prime mover 160, particularly by a singleprime mover such as a gas turbine or another internal combustion engine.In other words, each compressor stage of the plurality of compressorstages of the compressor 150 may be driven directly or indirectly by theprime mover 160. For example, a transmission mechanism, particularly agear of the compressor, with a plurality of gear wheels and/or othertransmission units such as pinions and/or pulleys may be connectedbetween the prime mover 160 and the plurality of compressor stages ofthe compressor 150, in order to drive the plurality of compressor stagesat appropriate rotational speeds.

In some embodiments, the compressor 150 includes a plurality of firstcompressor stages 151 configured for pressurizing the first refrigerantcirculating in the pre-cooling loop 110. For example, four firstcompressor stages may be provided. In other embodiments, a differentnumber of first compressor stages may be provided, e.g. two, three, ormore than four first compressor stages.

The plurality of first compressor stages 151 may be sequentiallyarranged in the pre-cooling loop. For example, the first refrigerantwhich enters the compressor 150 at an initial first compressor stage,may be subsequently pressurized by said initial first compressor stageand by other first compressor stage(s) arranged downstream from theinitial first compressor stage. The pressure of the first refrigerantmay be increased in each of the sequentially arranged first compressorstages 151.

In some embodiments, which may be combined with other embodimentsdescribed herein, the compressor 150 may include a plurality of secondcompressor stages 155 configured for pressurizing the second refrigerantcirculating in the cooling loop 130. For example, two, three, four ormore second compressor stages 155 may be provided.

The second compressor stages 155 may be sequentially arranged in thecooling loop. In other words, the second refrigerant which enters thecompressor 150 at an initial second compressor stage may be subsequentlypressurized by said initial second compressor stage and by furthersecond compressor stage(s) arranged downstream from the initial secondcompressor stage. The pressure of the second refrigerant may beincreased by each of the sequentially arranged second compressor stages155. The impellers of two or more second compressor stages may bemounted on different shafts and may be rotated at different rotationalspeeds in some embodiments.

For example, the compressor 150 may include four first compressor stagesfor pressurizing the first refrigerant and three (or alternatively four)second compressor stages for pressurizing the second refrigerant.

In some embodiments, the pre-cooling loop 110 may be configured todivide the first refrigerant into a plurality of precooling streams,which are guided to a respective one of said plurality of firstcompressor stages 151. The number of precooling streams may correspondto the number of first compressor stages. Each of the precooling streamsmay enter the compressor at an associated first compressor stage to bere-compressed by the associated first compressor stage and potentiallyby further first compressor stage(s) arranged downstream thereof, ifany.

In some embodiments, a plurality of first expansion elements 241, 243,245, 247 may be sequentially arranged in the pre-cooling loop 110 andconfigured for expanding the first refrigerant at a plurality ofdecreasing pressure levels. A plurality of first heat exchangers 249,251, 253, 255 of the first heat exchanger device 270 may be provided forreceiving respective precooling streams of said first refrigerantexpanded through at least one of said plurality of first expansionelements 241, 243, 245, 247 and for transferring heat from the naturalgas to the first refrigerant.

A plurality of return paths 261, 263, 265, 267 configured for returningsaid precooling streams of the first refrigerant from the plurality offirst heat exchangers 249, 251, 253, 255 to a respective one of saidplurality of first compressor stages 151 may be provided.

According to some embodiments, which may be combined with otherembodiments described herein, at least one first auxiliary expansionelement may be arranged in the precooling loop. Further, at least onefirst auxiliary heat exchanger may be provided for receiving at least aportion of said first refrigerant expanded through the at least onefirst auxiliary expansion element and for transferring heat from thesecond refrigerant to the first refrigerant.

According to some embodiments, which may be combined with otherembodiments described herein, the system may include a plurality offirst auxiliary expansion elements 221, 223, 225, 227 sequentiallyarranged in the pre-cooling loop 110 and configured for expanding thefirst refrigerant at a plurality of decreasing pressure levels. Aplurality of first auxiliary heat exchangers 229, 231, 233, 235 of thefirst heat exchanger device 270 may be provided for receiving respectiveportions of said first refrigerant expanded through at least one of saidplurality of first auxiliary expansion elements 221, 223, 225, 227 andfor transferring heat from the second refrigerant to the firstrefrigerant.

The plurality of return paths 261, 263, 265, 267 may be configured forreturning said portions of the first refrigerant from the plurality offirst auxiliary heat exchangers 229, 231, 233, 235 and/or from the firstheat exchangers 249, 251, 253, 255 to a respective one of said pluralityof first compressor stages 151.

During operation of the natural gas liquefaction system 200, a flow ofcompressed first refrigerant may be delivered from a most downstreamfirst compressor stage of the plurality of first compressor stages 151to a first condenser 17. The flow of the first refrigerant deliveredthrough the first condenser 17 may be cooled, e.g. against water or air,and condensed.

In some embodiments, the condensed first refrigerant is circulated inthe pre-cooling loop 110 to pre-cool the natural gas in the plurality offirst heat exchangers 249, 251, 253, 255, and/or to cool and optionallypartially liquefy the second refrigerant circulating in the cooling loop130 in the plurality of first auxiliary heat exchangers 229, 231, 233,235.

In some embodiments, the pre-cooling loop 110 may be divided into aplurality of n pressure levels, e.g. four pressure levels. The number nof pressure levels may correspond to the number n of first compressorstages of the compressor 150 configured for compressing the firstrefrigerant. The flow of first refrigerant delivered through the firstcondenser 17 may be sequentially expanded at n progressively reducingpressure levels and be divided into n partial flows. Each partial flowof first refrigerant may be returned as a side flow to the compressor150 at an inlet of a corresponding one of the plurality of firstcompressor stages 151.

A first delivery line 217 may deliver a first part of the condensedfirst refrigerant flow to the plurality of first expansion elements 241,243, 245, 247. A second delivery line 218 branched off the firstdelivery line 217 may deliver a second part of the condensed firstrefrigerant flow to the plurality of first auxiliary expansion elements221, 223, 225, 227.

The first part of the condensed first refrigerant from the firstcondenser 17 may be sequentially expanded in the plurality of firstexpansion elements 241, 243, 245, 247 at n different, graduallydecreasing pressure levels. Downstream from each first expansionelement, a portion of the flow of partly expanded first refrigerant maybe diverted to a respective one of the plurality of first heatexchangers 249, 251, 253, 255. The remaining part of the partly expandedfirst refrigerant may be caused to flow through the next first expansionelement and so on. The residual first refrigerant flowing through themost downstream one (247) of the plurality of first expansion elements241, 243, 245, 247 may be delivered to a most downstream one (255) ofthe plurality of first heat exchangers 249, 251, 253, 255.

In each one of the plurality of first heat exchangers 249, 251, 253,255, the first refrigerant may exchange heat against the natural gasflowing in the main natural gas line 61, thus pre-cooling and optionallypartly liquefying the natural gas.

The second part of the condensed first refrigerant expanded in at leastone of the plurality of first auxiliary expansion elements 221, 223,225, 227 may be diverted towards a corresponding one of the plurality offirst auxiliary heat exchangers 229, 231, 233, 235. The portion of firstrefrigerant delivered by each one of the plurality of first auxiliaryexpansion elements 221, 223, 225, 227 and which is not caused to flowthrough the respective first auxiliary heat exchanger is deliveredthrough the subsequent one of the plurality of first auxiliary expansionelements 221, 223, 225, 227. The most downstream one (235) of saidplurality of first auxiliary heat exchangers 229, 231, 233, 235 receivesthe residual fraction of first refrigerant expanded in the mostdownstream one (227) of the plurality of first auxiliary expansionelements 221, 223, 225, 227. In each first auxiliary heat exchanger, thefirst refrigerant exchanges heat against the second refrigerant whichcirculates in the cooling loop 130, so that at the delivery side of themost downstream one (235) of the plurality of first auxiliary heatexchangers 229, 231, 233, 235, the second refrigerant is cooled andoptionally at least partly liquefied.

Heated first refrigerant exiting the plurality of first heat exchangers249, 251, 253, 255 may be collected with the heated first refrigerantexiting the first auxiliary heat exchangers 229, 231, 233, 235 and maybe fed again to the integrally-geared turbo-compressor 150 at the inletof the respective first compressor stage.

In some embodiments, the heated first refrigerant exiting one of theplurality of first auxiliary heat exchangers 229, 231, 233, 235 is ataround the same pressure as the heated first refrigerant exiting acorresponding one of the plurality of first heat exchangers 249, 251,253, 255. The first refrigerant collected at corresponding pressurelevels may be delivered at the inlet of a corresponding stage of theplurality of first compressor stages of the compressor 150. A pluralityof side streams of the first refrigerant is thus returned at graduallydecreasing pressure levels at the inlets of the sequentially arrangedfirst compressor stages 151.

In some embodiments, the plurality of return paths 261, 263, 265, 267may be configured for delivering the side streams of expanded andexhausted first refrigerant from the plurality of first heat exchangers249, 251, 253, 255 and/or from the plurality of first auxiliary heatexchangers 229, 231, 233, 235 to a corresponding stage of the pluralityof first compressor stages 151.

In some embodiments, which may be combined with other embodimentsdescribed herein, the second refrigerant circulating in the cooling loop130 may be compressed by the plurality of second compressor stages 155which may be sequentially arranged in the cooling loop 130. Theplurality of second compressor stages 155 are part of the sameintegrally-geared compressor as the plurality of first compressor stages151.

In some embodiments, the integrally-geared compressor may include atleast one multi-stage compressor unit with two or more compressor stagessequentially arranged on a single shaft, e.g. a multi-stage centrifugalcompressor unit.

The prime mover 160 which drives the compressor may include an internalcombustion engine or an electric motor. The prime mover 160 can be a gasturbine, e.g. an aeroderivative gas turbine.

In some embodiments, at least one first intercooler may be arrangedbetween at least two sequentially arranged first compressor stages ofthe plurality of first compressor stages 151. In some embodiments, atleast one second intercooler may be arranged between at least twosequentially arranged second compressor stages of the plurality ofsecond compressor stages 155. The intercoolers may be configured toreduce the temperature and the volume of the respective refrigerantdelivered by the respective compressor stage before entering thesubsequent compressor stage or before leaving the compressor.

The second refrigerant delivered by the most downstream one of theplurality of second compressor stages 155 may be condensed by a secondcondenser 11. The second condenser 11 may be an air condenser of a watercondenser, where the second refrigerant may be condensed by exchangingheat against air or water. The condensed second refrigerant may besubsequently delivered by a delivery line through the plurality of firstauxiliary heat exchangers 229, 231, 233, 235, where the secondrefrigerant may be cooled and optionally liquefied by exchanging heatagainst the first refrigerant circulating in the pre-cooling loop 110,as described above.

The cooled second refrigerant delivered from the plurality of firstauxiliary heat exchangers may be guided toward the second heat exchangerdevice 180, which may be a main cryogenic heat exchanger, where thesecond refrigerant may remove further heat from the pre-cooled naturalgas, completing the liquefaction process. The heated second refrigerantmay be returned through a return line 269 to an initial one of theplurality of second compressor stages 155 of the compressor 150.

In FIG. 3, the plurality of compressor stages of the integrally-gearedturbo-compressor 150 is depicted in a schematic way only. The compressor150 of an exemplary embodiment is illustrated in more detail in FIG. 4.

FIG. 4 is an enlarged schematic view of a compressor arrangement with anintegrally-geared turbo-compressor 150 according to embodimentsdescribed herein. The compressor 150 may be driven by a prime mover 160and may include a plurality of compressor stages which are directly orindirectly driven by the prime mover 160. The plurality of compressorstages includes one or more first compressor stages 151 for pressurizingthe first refrigerant circulating in the pre-cooling loop 110, and oneor more second compressor stages 155 for pressurizing the secondrefrigerant circulating in the cooling loop 130. More details of thepre-cooling loop 110 and of the cooling loop 130 are described abovewith reference to FIG. 2 and FIG. 3 and are not repeated here.

The compressor 150 may include a transmission mechanism 301, e.g. anintegral gear, which may be arranged in a compressor housing 330 andconfigured to be driven by said prime mover 160. The compressor 150 mayfurther include at least one first shaft 303 configured to be driveninto rotation by said transmission mechanism 301 and configured fordriving at least one of the plurality of first compressor stages 151. Inother words, an impeller of at least one first compressor stage may bemounted on the at least one first shaft 303 such as to rotate togetherwith the first shaft. Further, the compressor 150 may include at leastone second shaft 305 configured to be driven into rotation by saidtransmission mechanism 301 and configured for driving at least one ofthe plurality of second compressor stages 155. Therein, an impeller ofat least one second compressor stage may be mounted on the at least onesecond shaft 305 such as to rotate together with the second shaft.

In some embodiments, at least one first shaft 303 may drive two firstcompressor stages of the plurality of first compressor stages, e.g. twosubsequent first compressor stages. Alternatively or additionally, atleast one second shaft 305 may drive two second compressor stages of theplurality of second compressor stages, for example two subsequent secondcompressor stages.

In some embodiments, the at least one first shaft 303 may be providedwith a pinion meshing with a gear wheel of the transmission mechanism301, and/or the at least one second shaft 305 may be provided with afurther pinion meshing with a gear wheel of the transmission mechanism301. For example, in some embodiments, the transmission mechanism 301may include a first gear wheel 307 configured for driving the at leastone first shaft 303 and a second gear wheel 308 configured for drivingthe at least one second shaft 305.

Alternatively, e.g. in the embodiment schematically depicted in FIG. 5,the transmission mechanism 301 may include one center gear wheel 307configured for driving the at least one first shaft 303 and for drivingthe at least one second shaft 305. For example, a single bull gear canbe provided that is configured for (e.g., directly) driving each of thefirst and second compressor stages.

In other words, a first pinion with a first diameter may be connected tothe at least one first shaft 303 and/or a second pinion with a seconddiameter may be connected to the at least one second shaft 305. Thecentral gear wheel 307 of the gear may directly mesh with the firstpinion and with the second pinion for driving the at least one firstshaft and the at least one second shaft into rotation. In the embodimentdepicted in FIG. 5, the central gear wheel 307 directly meshes withrespective pinions connected to two or more first shafts 303 and to twoor more second shafts 305. For example, the (single) central gear wheelmay directly drive the shafts of three, four or more first compressorstages and of three, four or more second compressor stages.

The first diameter of the first pinion may correspond to the seconddiameter of the second pinion. Accordingly, the first shaft and thesecond shaft may rotate at corresponding rotational speeds.Alternatively, the first diameter and the second diameter may bedifferent. Accordingly, the rotational speed of the first shaft and ofthe second shaft may be adjusted to differ as appropriate. For example,the rotational speeds of the first and second compressor stages may beadapted to the properties of the respective refrigerant guidedtherethrough.

In an alternative embodiment, two or more bull gears may be provided fordriving the plurality of compressor stages. For example, a first bullgear may drive the one or more first compressor stages, and a secondbull gear may drive the one or more second compressor stages.

It is noted that, in some embodiments, the at least one first shaftand/or the at least one second shaft may drive two compressor stageswhich may be arranged on opposite ends of the respective shaft. In FIG.3 and in FIG. 5, two compressor stages provided on a single shaft areschematically illustrated by two arrowheads directed in oppositedirections which are connected by a connection line illustrating thecommon shaft. For example, a first impeller of one compressor stage maybe mounted in a first portion of a common shaft, and a second impellerof a further compressor stage may be mounted in a second portion of thecommon shaft.

Referring back to FIG. 3 and to FIG. 4, in some embodiments, which maybe combined with other embodiments described herein, the first gearwheel 307 may drive the plurality of first compressor stages 151, andthe second gear wheel 308 may drive the plurality of second compressorstages 155. The gear wheels may be toothed wheels which are driven inrotation directly or indirectly by the prime mover 160, respectively.The first and second shafts may each comprise a pinion mounted thereonand meshing with a respective toothed wheel. The first and second shaftsand the impeller(s) mounted on the shafts can therefore rotate atdifferent rotational speeds.

The diameter of the second gear wheel 308 may be smaller than thediameter of the first gear wheel 307. When the first gear wheel 307directly meshes with the second gear wheel 308, the second gear wheel308 may rotate at a higher rotational speed than the first gear wheel307. Accordingly, the at least one second shaft 305 driven by the secondgear wheel 308 may be rotated at a higher rotational speed than the atleast one first shaft 303 driven by the first gear wheel 307. Thus, theimpeller(s) of the first compressor stage(s) which are mounted on the atleast one first shaft 303 may be rotated at a higher rotational speedthan the impeller(s) of the second compressor stage(s) which are mountedon the at least one second shaft 305.

In some embodiments, which may be combined with other embodimentsdescribed herein, the compressor may include two or more first shaftswhich drive the plurality of first compressor stages 151, wherein thetwo or more first shafts may be driven by the first gear wheel 307. Atleast one first shaft may be configured to drive two sequentiallyarranged first compressor stages. Alternatively or additionally, atleast one first shaft may be configured to drive a single firstcompressor stage. In the latter case, the impeller of a single firstcompressor stage may be mounted on the first shaft.

In some embodiments, which may be combined with other embodimentsdescribed herein, the compressor may include two or more second shaftsfor driving the plurality of second compressor stages 155, wherein thetwo or more second shafts may be driven by the second gear wheel 308. Atleast one second shaft may be configured to drive two sequentiallyarranged second compressor stages. Alternatively or additionally, atleast one second shaft may be configured to drive a single one of theplurality of second compressor stages.

Each compressor stage of the plurality of compressor stages may includea gas inlet, a gas outlet, and at least one impeller mounted on arespective shaft. Each impeller can be a radial impeller, with an axialinlet and a radial outlet. The fluid processed through the impeller maybe collected in a respective volute of the compressor stage. Theimpellers can be paired, wherein a pair of impellers (e.g. belonging totwo subsequent compressor stages) may be mounted on a common rotaryshaft.

In some embodiments, the plurality of first compressor stages 151 may beconfigured to compress the first refrigerant so that the pressurizedfirst refrigerant is delivered from the most downstream first compressorstage 312 of the plurality of first compressor stages 151 at a pressureranging from 10 bar to 40 bar absolute, particularly from 20 bar to 30bar absolute, more particularly from 22 bar to 24 bar absolute. Thepressure of the first refrigerant at the inlet of the most upstreamfirst compressor stage 315 may be between 1 bar absolute and 2 barabsolute in some embodiments.

Alternatively or additionally, the plurality of first compressor stages151 may be configured to compress the first refrigerant so that thepressurized first refrigerant is delivered from the most downstreamfirst compressor stage 312 of the plurality of first compressor stages151 at a temperature ranging from 60° C. to 100° C., particularly from75° C. to 85° C. For example, no inter-cooling stage may be providedbetween the two first compressor stages.

In some embodiments, the at least one first shaft 303 on which one ormore impellers of one or more first compressor stages 151 are mountedmay be configured to rotate at a rotational speed from 3.000 rpm(rotations per minute) to 7.000 rpm, particularly from about 4.000 rpmto about 5.500 rpm. In some embodiments, two or more first shafts may beprovided, on which the impellers of all of the first compressor stagesare mounted. Each first shaft may be configured to rotate at arotational speed from 3000 rpm to about 7000 rpm. The shaft of the mostupstream first compressor stage may rotate at a lower speed than theshaft of the most downstream first compressor stage.

The plurality of first compressor stages 151 may deliver the compressedfirst refrigerant at a flow rate ranging from about 10,000 actual m³/hto about 70,000 actual m³/h.

The plurality of first compressor stages 151 may absorb a power rangingfrom about 10 MW to about 40 MW, particularly ranging from about 25 MWto about 35 MW. Alternatively or additionally, the plurality of secondcompressor stages 155 may absorb a power ranging from about 10 MW toabout 40 MW, particularly ranging from about 25 MW to about 35 MW.Accordingly, in some embodiments, the prime mover 160 may provide apower ranging from 20 MW to 80 MW, particularly from 50 MW to 70 MW.

In some embodiments, the plurality of second compressor stages 155 maybe configured to compress the second refrigerant so that the pressurizedsecond refrigerant is delivered from the most downstream secondcompressor stage 316 of the plurality of second compressor stages 155 ata pressure ranging from 50 bar to 100 bar absolute, particularly from 55bar to 65 bar absolute. The pressure of the second refrigerant at theinlet of the most upstream second compressor stage 319 may be below 10bar absolute in some embodiments.

In some embodiments, the plurality of second compressor stages 155 maybe configured to compress the second refrigerant so that the pressurizedsecond refrigerant is delivered from the most downstream secondcompressor stage 316 of the plurality of second compressor stages 155 ata temperature ranging from 60° C. to 120° C., particularly from 80° C.to 100° C. For example, one, two or more inter-cooling stages 320 may beprovided between at least two subsequent second compressor stages. Thus,the exit temperature of the second refrigerant can be reduced.

The at least one second shaft 305 on which the impeller(s) of one ormore second compressor stages is mounted may be configured to rotate ata rotational speed from 7.000 rpm to 20.000 rpm, particularly from 8.000rpm to about 15.000 rpm. In some embodiments, two or more second shaftsmay be provided for driving the impellers of all second compressorstages 155. The shaft of the most upstream second compressor stage 319may rotate at a lower speed (e.g. between 9.000 rpm and 11.000 rpm) thanthe shaft of the most downstream second compressor stage 316 (e.g. at aspeed between 14.000 rpm and 16.000 rpm).

FIG. 4 shows an exemplary embodiment, in which the plurality of firstcompressor stages 151 includes a total of four subsequently arrangedfirst compressor stages. The upstream pair of first compressor stages isdriven by a rotary shaft, and the downstream pair of first compressorstages is driven by a further rotary shaft, wherein both rotary shaftsare driven by the first gear wheel 307. In other words, the impellers ofthe upstream pair of first compressor stages are mounted on a commonrotary shaft, and the impellers of the downstream pair of firstcompressor stages are mounted on a further common rotary shaft.Alternatively, only the upstream pair of first compressor stages may bedriven by a common rotary shaft, whereas the two downstream firstcompressor stages may be driven by a separate rotary shaft,respectively, or vice versa.

In the exemplary embodiments of FIG. 4, the plurality of secondcompressor stages 155 includes a total of four subsequently arrangedsecond compressor stages. The upstream pair of second compressor stagesis driven by a rotary shaft, and the downstream pair of secondcompressor stages is driven by a further rotary shaft, wherein bothrotary shafts are driven by the second gear wheel 308. In other words,the impellers of the upstream pair of second compressor stages aremounted on a common rotary shaft, and the impellers of the downstreampair of second compressor stages are mounted on a further common rotaryshaft. Alternatively, only three subsequently arranged second compressorstages may be provided, wherein the upstream pair of second compressorstages may be driven by a common rotary shaft, and the downstream secondcompressor stage may be driven by a separate rotary shaft, or viceversa.

Other possible arrangements and numbers of first and second compressorstages on respective rotary shafts driven into rotation by thetransmission mechanism or gear of the compressor will be apparent to theskilled person.

According to a further aspect, a compressor arrangement for compressinga plurality of refrigerants is provided. The compressor arrangementincludes an integrally-geared turbo-compressor 150 with a plurality ofcompressor stages which may have some or all of the features of theabove described compressors.

The compressor arrangement may include a first cooling line, e.g. beingpart of the pre-cooling loop, through which a first refrigerant isadapted to flow, wherein one or more first compressor stages of theplurality of compressor stages are adapted to pressurize the firstrefrigerant streaming through the first cooling line. The compressorarrangement may further include a second cooling line, e.g. being partof the cooling loop, through which a second refrigerant is adapted toflow, wherein one or more second compressor stages of the plurality ofcompressor stages are adapted to pressurize the second refrigerantstreaming through the second cooling line.

The compressor arrangement may be used in a natural gas liquefactionsystem according to any of the embodiments described above.

The compressor arrangement may include a transmission mechanism or gearwith some or all of the features of the embodiments described above.Further, all compressor stages may be included in a single housing insome embodiments.

According to a further aspect described herein, a method of liquefying anatural gas is provided. A flow diagram of a method according toembodiments described herein is schematically depicted in FIG. 6.

In box 710, an integrally-geared turbo compressor having a plurality ofcompressor stages is provided. In box 720, the compressor is driven witha prime mover. In box 730, a first refrigerant is circulated through oneor more first compressor stages of the plurality of compressor stages,and a second refrigerant is circulated through one or more secondcompressor stages of the plurality of compressor stages. In box 740, atleast one of natural gas and the second refrigerant is cooled by heatexchange against the first refrigerant. In box 750, the natural gas iscooled by heat exchange against the second refrigerant.

In some embodiments, the compressed first refrigerant and/or thecompressed second refrigerant may be condensed. The condensed firstrefrigerant may be expanded, e.g. in a plurality of sequentiallyarranged first expansion elements.

In some embodiments, the first refrigerant may be divided in a pluralityof partial flows.

In some embodiments, at least a part of the first refrigerant may besequentially compressed by a plurality of first compressor stages, e.g.three, four or more first compressor stages, and/or the secondrefrigerant may be sequentially compressed by a plurality of secondcompressor stages, e.g. three, four or more second compressor stages.

Movable inlet guide vanes may be provided at inlets of at least one ofthe plurality of first compressor stages. The movable inlet guide vanesmay be individually controlled to regulate partial flows at the suctionside of the plurality of first compressor stages, particularly as afunction of flow conditions of the partial flows.

In some embodiments, the method may further include: expanding the firstrefrigerant through a plurality of sequentially arranged first expansionelements at a plurality of decreasing pressure levels; circulatingportions of the expanded first refrigerant from the first expansionelements through a plurality of first heat exchangers to remove heatfrom the natural gas; and returning the portions of expanded firstrefrigerant from the plurality of first heat exchangers to respectiveones of the one or more first compressor stages.

In some embodiments, the method may further include: expanding the firstrefrigerant through a plurality of sequentially arranged first auxiliaryexpansion elements at a plurality of decreasing pressure levels;circulating portions of the expanded first refrigerant through aplurality of first auxiliary heat exchangers to remove heat from thesecond refrigerant; and returning the portions of first refrigerant fromthe plurality of first auxiliary heat exchangers to a respective one ofthe one or more first compressor stages.

The prime mover may drive a transmission mechanism, e.g. an internalgear, of the compressor, wherein the transmission mechanism may drive atleast one first shaft and at least one second shaft into rotation.

The at least one first shaft may be driven into rotation by saidtransmission mechanism at a rotation speed of 3.000 rpm or more and7.000 rpm or less. The impellers of one or two first compressor stagesmay be mounted on the at least one first shaft and may rotate at therotational speed of the at least one shaft.

The at least one second shaft may be driven into rotation by saidtransmission mechanism at a rotation speed of 8.000 rpm or more and15.000 rpm or less and may drive at least one of the second compressorstages. In other words, an impeller of at least one second compressorstage may be mounted on the at least one second shaft.

In some embodiments, the first refrigerant may be sequentiallycirculated through three, four or more first compressor stages of thecompressor and compressed to an exit pressure ranging from 10 bar to 40bar absolute, particularly from 20 bar to 30 bar absolute.

In some embodiments, the second refrigerant may be sequentiallycirculated through three, four or more second compressor stages of thecompressor and compressed to an exit pressure ranging from 40 bar to 100bar absolute, particularly from 50 bar to 80 bar absolute.

The use of an integrally-geared turbo-compressor for pressurizing two ormore different refrigerants circulating in two or more cooling loops mayresult in an enhanced efficiency of the natural gas liquefaction systemand thus reduced power consumption, and may further result inconsiderable cost savings when compared to systems with two or moreseparate compressors and compressor driving units. Further, the gear ofthe compressor may be adjusted such that each compressor stage mayrotate at an appropriate rotation speed. Using a single compressor unitin a natural gas liquefaction system is an advantage in terms of cost,footprint and flexibility.

The number of first and second compressor stages as well as details ofthe internal gear of the compressor (e.g. details of the transmissionmechanism) may depend on the properties of the refrigerants to becompressed. Further, a larger or a modified transmission mechanism maybe provided, if three, four or more refrigerants are to be compressed bythe integrally-geared compressor.

While the foregoing is directed to embodiments of the disclosure, otherand further embodiments of the disclosure may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A natural gas liquefaction system (100), comprising: anintegrally-geared turbo-compressor (150) with a plurality of compressorstages; a single prime mover (160) for driving the compressor (150); apre-cooling loop (110), through which a first refrigerant is adapted tocirculate, wherein one or more first compressor stages (151) of theplurality of compressor stages are adapted to pressurize the firstrefrigerant; a cooling loop (130), through which a second refrigerant isadapted to circulate, wherein one or more second compressor stages (155)of the plurality of compressor stages are adapted to pressurize thesecond refrigerant; a first heat exchanger device (170) for transferringheat from a natural gas and/or from the second refrigerant to the firstrefrigerant; and a second heat exchanger device (180) for transferringheat from the natural gas to the second refrigerant; wherein said singleprime mover (160) drives each of the one or more first compressor stages(151) and second compressor stages (155).
 2. The system of claim 1,wherein the compressor (150) comprises a plurality of first compressorstages (151), particularly four sequentially arranged first compressorstages, for pressurizing the first refrigerant, and/or a plurality ofsecond compressor stages (155), particularly three or four sequentiallyarranged second compressor stages, for pressurizing the secondrefrigerant.
 3. The system of claim 1, wherein the compressor (150)comprises: a transmission mechanism (301), particularly including agear, configured to be driven into rotation by said prime mover; atleast one first shaft (303) configured to be driven into rotation bysaid transmission mechanism (301) and configured for driving at leastone of the first compressor stages; and at least one second shaft (305)configured to be driven into rotation by said transmission mechanism(301) and configured for driving at least one of the second compressorstages.
 4. The system of claim 3, wherein the transmission mechanism(301) comprises a first gear wheel (307) meshing at least one firstpinion connected to the at least one first shaft (303) for driving theat least one first compressor stage.
 5. The system of claim 3, whereinthe first gear wheel (307) further meshes at least one second pinionconnected to the at least one second shaft (305) for driving the atleast one second compressor stage.
 6. The system of claim 3, wherein thetransmission mechanism (301) comprises a first gear wheel (307)configured for driving the at least one first shaft (303), and a secondgear wheel (308) configured for driving the at least one second shaft(305), particularly wherein the diameter of the second gear wheel (308)is smaller than the diameter of the first gear wheel (307) and/orwherein the first gear wheel and the second gear wheel are directlymeshing gear wheels.
 7. The system of claim 3, wherein at least one ofthe at least one first shaft and the at least one second shaft drivestwo compressor stages arranged on opposite ends of the respective shaft.8. The system of claim 1, wherein the compressor (150) comprises aplurality of first compressor stages (151), and wherein the pre-coolingloop (110) is configured to divide the first refrigerant into aplurality of precooling streams, which are guided to a respective one ofsaid plurality of first compressor stages (151).
 9. The system of claim8, comprising: a plurality of first expansion elements (241, 243, 245,247) sequentially arranged in the pre-cooling loop (110) and configuredfor expanding the first refrigerant at a plurality of decreasingpressure levels; a plurality of first heat exchangers (249, 251, 253,255) of the first heat exchanger device (170, 270) for receivingrespective precooling streams of the first refrigerant expanded throughat least one of said plurality of first expansion elements (241, 243,245, 247) and for transferring heat from the natural gas to the firstrefrigerant; and a plurality of return paths (261, 263, 265, 267)configured for returning said precooling streams of the firstrefrigerant from the plurality of first heat exchangers (249, 251, 253,255) to a respective one of the plurality of first compressor stages(151).
 10. The system of claim 1, comprising at least one firstauxiliary expansion element arranged in the pre-cooling loop (110) andat least one first auxiliary heat exchanger of the first heat exchangerdevice (170, 270) configured for receiving a portion of said firstrefrigerant expanded through the at least one first auxiliary expansionelement and for transferring heat from the second refrigerant to thefirst refrigerant.
 11. The system of claim 10, comprising: a pluralityof first auxiliary expansion elements (221, 223, 225, 227) sequentiallyarranged in the pre-cooling loop (110) and configured for expanding thefirst refrigerant at a plurality of decreasing pressure levels; aplurality of first auxiliary heat exchangers (229, 231, 233, 235) of thefirst heat exchanger device (170, 270) configured for receivingrespective portions of said first refrigerant expanded through at leastone of said plurality of first auxiliary expansion elements (221, 223,225, 227) and for transferring heat from the second refrigerant to thefirst refrigerant; and a plurality of return paths (261, 263, 265, 267)configured for returning said portions of the first refrigerant from theplurality of first auxiliary heat exchangers (229, 231, 233, 235) to arespective one of said plurality of first compressor stages (151). 12.The system of claim 1, wherein the first refrigerant comprises a gaswith a molecular weight of 40 or more, particularly propane, and/orwherein the second refrigerant is a mixed refrigerant, particularly amixture comprising methane, ethane, propane and/or nitrogen.
 13. Thesystem claim 1, wherein said prime mover (160) comprises an electricmotor or an internal combustion engine, particularly a gas turbine. 14.A method of liquefying natural gas, comprising: providing anintegrally-geared turbo-compressor (150) having a plurality ofcompressor stages; driving the compressor (150) with a single primemover (160); circulating a first refrigerant through one or more firstcompressor stages (151) of the plurality of compressor stages, each ofthe one or more first compressor stages (151) being driven by saidsingle prime mover (160); circulating a second refrigerant through oneor more second compressor stages (155) of the plurality of compressorstages, each of the one or more second compressor stages (155) beingdriven by said single prime mover (160); cooling at least one of naturalgas and the second refrigerant by heat exchange against the firstrefrigerant; and cooling the natural gas by heat exchange against thesecond refrigerant.
 15. The method of claim 14, further comprising:expanding the first refrigerant through a plurality of sequentiallyarranged first expansion elements (241, 243, 245, 247) at a plurality ofdecreasing pressure levels; circulating portions of the firstrefrigerant from the plurality of sequentially arranged first expansionelements through a plurality of first heat exchangers (249, 251, 253,255) to remove heat from the natural gas; and returning the portions ofthe first refrigerant from the plurality of first heat exchangers torespective ones of said one or more first compressor stages.
 16. Themethod of claim 14, wherein a transmission mechanism (301) of thecompressor is driven by the prime mover (160), at least one first shaft(303) is driven into rotation by said transmission mechanism (301) at arotation speed of 3.000 rpm or more and 7.000 rpm or less and drives atleast one of the first compressor stages; and at least one second shaft(305) is driven into rotation by said transmission mechanism (301) at arotation speed of 8.000 rpm or more and 20.000 rpm or less and drives atleast one of the second compressor stages.
 17. The method of claim 14,wherein the first refrigerant is sequentially circulated through three,four or more first compressor stages and compressed to an exit pressureranging from 10 bar to 40 bar absolute, and/or wherein the secondrefrigerant is sequentially circulated through three, four or moresecond compressor stages and compressed to an exit pressure ranging from50 bar to 100 bar absolute.
 18. The method of claim 14, furthercomprising controlling independently movable inlet guide vanes toregulate partial flows at a suction side of the one or more firstcompressor stages, particularly as a function of flow conditions ofrespective partial flows.